The sea cucumber (Apostichopus japonicus) occupies a basal position during the evolution of deuterostomes and is also an important aquaculture species. In order to identify more immune effectors, transcriptome sequencing of A. japonicus coelomocytes in response to lipopolysaccharide (LPS) challenge was performed using the Illumina HiSeq™ 2000 platform. One hundred and seven differentially expressed genes were selected and divided into four functional categories including pathogen recognition (25 genes), reorganization of cytoskeleton (27 genes), inflammation (41 genes) and apoptosis (14 genes). They were analyzed to elucidate the mechanisms of host-pathogen interactions and downstream signaling transduction. Quantitative real-time polymerase chain reactions (qRT-PCRs) of 10 representative genes validated the accuracy and reliability of RNA sequencing results with the correlation coefficients from 0.88 to 0.98 and p-value <0.05. Expression analysis of immune-related genes after LPS challenge will be useful in understanding the immune response mechanisms of A. japonicus against pathogen invasion and developing strategies for resistant markers selection.

Sea cucumber Apostichopus japonicus is an important economic species in China, which is affected by various diseases; skin ulceration syndrome (SUS) is the most serious. In this study, we characterized the transcriptomes in A. japonicus challenged with Vibrio splendidus to elucidate the changes in gene expression throughout the three stages of SUS progression. RNA sequencing of 21 cDNA libraries from various tissues and developmental stages of SUS-affected A. japonicus yielded 553 million raw reads, of which 542 million high-quality reads were generated by deep-sequencing using the Illumina HiSeq™ 2000 platform. The reference transcriptome comprised a combination of the Illumina reads, 454 sequencing data and Sanger sequences obtained from the public database to generate 93,163 unigenes (average length, 1,052 bp; N50 = 1,575 bp); 33,860 were annotated. Transcriptome comparisons between healthy and SUS-affected A. japonicus revealed greater differences in gene expression profiles in the body walls (BW) than in the intestines (Int), respiratory trees (RT) and coelomocytes (C). Clustering of expression models revealed stable up-regulation as the main pattern occurring in the BW throughout the three stages of SUS progression. Significantly affected pathways were associated with signal transduction, immune system, cellular processes, development and metabolism. Ninety-two differentially expressed genes (DEGs) were divided into four functional categories: attachment/pathogen recognition (17), inflammatory reactions (38), oxidative stress response (7) and apoptosis (30). Using quantitative real-time PCR, twenty representative DEGs were selected to validate the sequencing results. The Pearson’s correlation coefficient (R) of the 20 DEGs ranged from 0.811 to 0.999, which confirmed the consistency and accuracy between these two approaches. Dynamic changes in global gene expression occur during SUS progression in A. japonicus. Elucidation of these changes is important in clarifying the molecular mechanisms associated with the development of SUS in sea cucumber.

Excessive reactive oxygen species (ROS) may lead to oxidative damage and metabolic disorder. The pathogenesis of human bowel inflammation is closely related to oxidative damage of intestinal epithelial cells caused by ROS. This study aimed to explore the high-value utilization of the byproducts of sea cucumber in antioxidant food for colitis prevention. The technology of protease hydrolysis combined with Cordyceps militaris fermentation was used to obtain fermented sea cucumber viscera protease hydrolysates (FSVHs). The results revealed that FSVH could enhance antioxidant capacity and alleviate oxidative damage and apoptosis by activating the Nrf2/HO-1 pathway and triggering the self-protection immune mechanisms. Moreover, the FSVH supplementation could upregulate antioxidant-related metabolic pathways of Caco-2 cells such as glutathione metabolism, confirming the enhanced antioxidant capacity of damaged cells. In summary, FSVH could exert protective effects on Caco-2 cells in response to oxidative damage, providing a promising prospect for sea cucumber resource utilization and colitis prevention.

Apostichopus japonicus is one of the few animals that can synthesize saponins. Present understanding of saponin biosynthesis is often limited to genome analysis, leaving the precise mechanism still unclear. This study examined the potential role of a gene annotated as farnesyl pyrophosphate synthase (FPS) in saponin biosynthesis of A. japonicus. We amplified its complete cDNA (denoted as AjFPS) and produced its recombinant protein (rAjFPS) through rapid amplification of cDNA ends (RACE) and Escherichia coli expression system, respectively. RNA interference and correlation analysis of AjFPS transcriptional expression level and saponin content were conducted to evaluate the role of AjFPS in saponin biosynthesis. A pull‐down assay was used to preliminarily explore its regulatory role in saponin biosynthesis. The complete cDNA sequence of AjFPS was shown to be 3,006 bp in length. The inferred amino acid sequence harbored two conserved aspartate‐rich sites (DDXXD). Analysis of the amino acid sequence using phylogenetics showed that AjFPS was more similar to FPSs found in saponin‐producing marine organisms rather than those in non‐saponin‐producing ones or plants. The knockdown of AjFPS resulted in a remarkable decline in saponin content. The temporal expression of AjFPS was significantly correlated with the variation in saponin content after Vibrio splendidus challenge. The pull‐down analysis illustrated that AjFPS interacted directly or indirectly with diverse immune‐related factors associated with pathogen recognition and destruction and protection against oxidative stress. Combined, these results suggested that AjFPS is involved in saponin synthesis, and this process in A. japonicus is likely modulated by immune processes, with AjFPS serving as a regulatory node. These findings will promote a deeper understanding of saponin synthesis and immune regulation as well as contribute to the improvement of saponin quality traits in cultured A. japonicus.